Boxing Clever: Crossrail at Paddington

91 comments

With the euphoria surrounding the completion of Crossrail’s running tunnels and the subsequent dismantling of the TBMs it is easy to forget that there is an awful lot of civil engineering construction work still to be done on the project. In this article we look at the current state of work at Paddington Crossrail station.

Building large, underground stations in central London is always a difficult task. This is certainly true on Crossrail, the stations for which are generally being built to a scale not seen before in London. Even amongst these, however, Crossrail Paddington stands apart. For it has been constructed in a completely different manner from any other station built directly by the project for the new line.

Until the 1980s there was, essentially, only one way of building a deep level station. This involved the construction of two large tube tunnels for the platforms and the addition of multiple circular or near-circular connecting passages for passenger tunnels, stairs, escalators and lifts.

As civil engineering in general has developed, new techniques have been devised to allow construction on a larger scale. This can clearly been seen above ground in London with a plethora of new tall buildings using construction techniques that just weren’t around forty years ago. Less visible though, for obvious reasons, are similar advances beneath ground level.

Boxing clever

A relatively new technique for building underground stations, very much favoured on the Jubilee Line Extension, is the station box. An enormous rectangular concrete box is dug down from the surface into which all the platforms and other ancillary features are placed. When implemented this becomes a game changer. No longer does a station tend to have a cramped feel and it becomes much easier to install escalators, lifts and ancillary equipment that normally has to be squeezed into the limited space available. A feature typical of a station box is a single wide island platform serving both tracks – ideal as escalators, lifts and more can then conveniently serve all passengers regardless of which direction they are travelling in.

Paddington Box Overview

Overview of Paddington Station Box August 2015

The station box concept is not a panacea and has its limitations. To make use of this technique, it is necessary to acquire all of the land covered by the footprint of the box. It also produces a considerable amount of spoil. Cost can be a further consideration and if you do things on a larger scale it is unlikely to be cheap. This can be seen, for example, at Tooting Broadway on Crossrail 2, where adverse geological conditions would more or less force construction of a station box. This would likely double the cost of constructing the station as well as be much more disruptive for the local community.

A much less obvious problem with station boxes is that of settlement upwards. Despite the huge weight of the concrete sides and the contents, the greater weight of material extracted means that designers have to be careful that the concrete box does not develop a tendency to rise as the forces in the ground push it upward.

It is fairly obvious from the above that station boxes tend to be built on brownfield sites that are expecting substantial development. This was the case at Canary Wharf (Jubilee), Canada Water and North Greenwich. It could hardly be called a brownfield site, but at Canary Wharf (Crossrail) the developers considered it worthwhile to drain the dock in order to create a suitable site. Indeed Paddington Crossrail is to date the only example of a deep-level station box being built in central London, for although the same construction methods are being used at Farringdon, the station platform tunnels there will be “tube” tunnels linking the two separate entrances.

A near golden scenario

Normally when central Crossrail stations are being talked about, the constricted nature of the sites is mentioned. Even though the site at Tottenham Court Road may look large it is in fact, by construction standards, quite small and had to accommodate an awful lot of carefully sequenced concurrent activity.

At Paddington a combination of fortuitous circumstances made a station box both possible and desirable. The first of these, and by far the biggest factor, was that the space could be made available. Along the length of Paddington main line station is Eastbourne Terrace, a relatively wide road that was long, straight and – most importantly – not really a primary road. It is hard to imagine a better opportunity next to a main line terminus. Furthermore, almost miraculously, the site was unhampered by the four Underground lines that serve Paddington and also the Post Office Railway whose mothballed tunnels are nearby.

Things got better still as the main line station that Crossrail was to serve was already below street level. This meant that the road could be reinstated after the work was done and the connections with the main line station would not be in conflict with other activity on the surface. The fact that it was serving a main line station meant that the expense of the station box could also be justified by the projected passenger numbers.

What may well have been the icing on the cake for a station box was that the main location for removing spoil from the Crossrail tunnels by rail transfer was located about a mile away. Removing the spoil meant that there were up to 90 lorry movements per night. The short distance to the unloading area at Old Oak meant that multiple trips by one lorry could be made and its location, between a main line railway and a road that is an urban motorway in all but name, was one where there would hardly be an issue with nighttime lorry movements.

Problems before the start

Of course this did not make it a problem-free scenario. One issue was that Eastbourne Terrace was used for taxis to queue to pick up passengers from Paddington. There is quite a substantial demand for taxis at Paddington and it was not going to be simply a case of uprooting the pole indicating where to wait and relocating it in another side street. The issue of where to relocate the taxi queue was quite a challenge and the new location for the taxi rank is a substantial purpose-built affair that we have already covered in a separate article.

The issue of relocating the taxis was to exacerbate a problem that was already present. Ideally one builds the station tunnel first and then, on arrival, the Tunnel Boring Machines (TBMs) are transported from one end to the other. On Crossrail that saves around 250 metres of boring for each of the two running tunnels. It is also very useful as it gives an opportunity for the TBMs to be stopped and serviced – one of the cardinal rules of tunnelling is that, as much as possible, once you start tunnelling you keep going. Clearly getting the station box in a state that it would be ready to receive a TBM was not going to be possible so the alternative was to construct the running tunnel first then break through the newly installed tunnel rings afterwards.

A further problem was that an awful lot of other construction activity, largely unrelated, was taking place at Paddington. Roofs were being refurbished on the main line station and the Hammersmith & City station was being rebuilt. Ownership demarcation lines didn’t naturally align with logical construction demarcation lines. A co-ordinated programme was also essential to avoid other issues like conflicting lorry routes and to ensure that pedestrian routes were kept open. This was known as the Paddington Integrated Project.

The other inevitable problem was that underneath roads one tends to have an awful lot of services. One of the advantages of deep tunnels is that they avoid the difficulty of moving these services but, in the case of station boxes, one encounters the same problem that the original builders of the Metropolitan Railway had just yards from the site. The services in Eastbourne Terrace were relocated under the pavement of the south west side of the road – a process that took two years.

Why so Big?

Despite the opportunity there remains a need to ask whether the box method was financially beneficial, given a more conventionally built station would be able to achieve something quite adequate for lower cost. One critical element, however, made it worthwhile – fans.

paddingtoncross

Architect’s overview showing emergency evacuation staircases at each end of the station

Paddington station is a major site for managing air-flow – especially in the event of emergency. Even in normal conditions one has to bear in mind that the trains will have air-conditioning, so the tunnels are liable to get very hot in summer unless something is done to get rid of the heat. The enormous fans that will be located there will work in tandem with similar enormous fans at Farringdon to control the air in the tunnels. Associated with the fans are separate emergency access shafts that will be guaranteed to have air as fresh as the streets outside in the event of fire – very useful for emergency safe evacuation or as an access route for firefighters.

It is probably worth bearing in mind that, whether there was a station or not at Paddington, the enormous fans would still be necessary and really ought to be considered a tunnelling cost rather than a station cost. When this is taken into account the station box was probably actually the cheapest option.

The situation today

Fast forwarding to the present, the box has been built. This involved a full closure of Eastbourne Terrace whilst the major work was carried out and even now there are currently just two lanes. The road is also restricted to buses.

Plunge Column

Top of a temporary plunge column that will be a challenge to remove

This doesn’t mean work is nearly complete and there is much still to do. One of the major tasks coming to completion is to remove the “plunge columns”. These are the vertical metal beams that were necessary to hold up the roof covering on which the reopened Eastbourne Terrace now sits. Some are needed permanently and these have now been given a concrete pillar surround but many are temporary. Of these some can be cut away in one piece and lifted by crane onto a low-loader while others have to be cut away at the base into pieces, with the column being lowered between cuts. It is a major job but one that has been taken into account in the planning process. It is also a job that sometimes means it is necessary to close Eastbourne Terrace over a weekend.

At track level

A photo taken at track level that completely fails to show the length of the station

What is also difficult to grasp is the sheer size of the station box. Platformless, and without any platform edge doors to reduce the width, the sheer size is tricky to comprehend. Unfortunately, due to the amount of construction (and deconstruction) work going on it is not easy to convey this in a picture as it becomes almost impossible to get more than a glimpse of the far end when in the vicinity of the tunnel eyes.

Tunnel eyes

The tunnel eyes themselves are a truly impressive feature. It is not often that one is presented with a chance to look at a Crossrail running tunnel without actually being in it. On a lighter note, each tunnel eye is boarded off except for what looks like a small door. One almost expects to see a large bottle nearby with a label attached and the words “Drink Me” on the label.

tunneleyes

The tunnel eyes before construction work proper began

Small door in tunnel

Beyond the tunnel eye today

A station box is, by necessity, built using a “top down” approach. Basically one digs down as far as it is safe to do so then builds the side walls. The process is repeated many times with the existing side walls being underpinned at each stage. Obviously, this process cannot be continued when one gets to the location of the tunnel eyes in quite the same way so these were done last. In recent weeks the concrete around the tunnel eyes, 0.6m thick, has been poured into some incredibly substantial shuttering to complete the job. The amount of steel reinforcement in place just for this task gives some idea of the volumes of steel reinforcement that is necessary at sites like this.

Adding reinforcement to tunnel eye wall

Reinforcing the wall around the tunnel eye

Fitting Out

“Fitting Out” does not sound like a big task but with a station box 260m long and on multiple levels this is not a trivial matter. Even building the platforms is a major project involving more concrete pouring. Rectangular spaces are left at regular intervals as below the platforms will be some space for installing plant and these will need to be able to be accessed from the track.

Reinforcing bars for platform wall

Preparation for building platform wall

One thing that is going to be very different from the deep level tube is the distance from the platform surface to the running rails. By the time the base is taken into account it is clear that the platform wall adjacent to the track is going to be taller than the average person.

Test structure for platform edge doors

Test structure for Platform Edge Door construction

An ongoing challenge is that many of the tasks still to be executed will need to be done on a massive scale. Just tiling the platform floor or installing the platform edge doors over a length of half a kilometre (250m per platform) is going to be a major logistical task. A nine car train with three doors per car means 27 pairs of platform edge doors per platform. Even at this early stage, the methodology for installing the platform edge doors is being checked and verified. A very small section of ceiling mounted frame has already been installed.

Lighting

Concrete lampshades

Concrete “lampshades”

Lighting is a major consideration these days when designing a station. The Jubilee Line Extension pioneered the idea in London of using natural light when possible. This has been a fundamental part of the design at Paddington, but with platforms around 20m below the surface artificial light will also be necessary even in day time. To this end the lighting has been designed to take advantage of the concrete ceiling, but this means that there will be a lot of light fittings to install. The result should be very pleasing with the transition from natural light to artificial light scarcely noticeable. This success or otherwise will partly depend on the quality and suitability of the concrete for the specific task in hand. In general, throughout Crossrail, the quality of concrete is very high and it is tailored to the specific purpose it is being used for. In this particular case a very smooth shiny finish is absolutely essential.

paddingtonfinishedbox

Architect’s impression showing relationship between natural and artificial lighting

Stand clear! Slow train approaching

Obstructed route for MPG

All this will have to be clear before the Multipurpose Gantries can pass through the station

If fitting out was not enough of a challenge, the site’s organisers also need to bear in mind that any of the Multi-purpose gantries (MPGs) – basically a factory on wheels for various tasks such as tracklaying, concreting sleepers in, installing electric cabling including the overhead rigid power rail – need to be able to get through the station box on their way from Westbourne Park to Farringdon. Because these MPGs are too large to get through the crossover at Fisher Street, both platforms need to be clear to enable their progress. The logistics for this have not yet been finalised but with some of the fleet already delivered and one of the temporary logistics centres for this at nearby Westbourne Park, it will probably not be long before a clear passage through the tunnels and the station box is going to be necessary.

Deeper Still

Although the station box is around 20m deep it does not end there. In the original plans for Paddington there was to be a link to the Bakerloo line just below the surface of the concourse area of Paddington main line station. Further investigation of the ground and structures above it meant that this was not a viable option and so an alternative plan had to be put in place.

The alternative plan was to go very much deeper so that you went down from platform level at Paddington Crossrail into a cross passage that was around 100m long and ended underneath the area between the platforms of the Bakerloo line. This would enable it to come up to the area at the foot of the escalators that lead down to the Bakerloo platform.

Entrance to future Bakerloo Line Link tunnel

Future entrance to Bakerloo Line Link

Because it was not part of the plan that was incorporated into the Crossrail Act, the deep level Bakerloo line link was subject to a Transport and Works Act Order with all the delay that entails. For this reason there was, at one stage, concern that it would not be ready in time. Nevertheless, construction of the Bakerloo Line Link is now underway and it is due to finish well before December 2018 when Paddington Crossrail station opens.

Former Royal Mail Building London Street

An unlikely Crossrail construction site

The bulk of the work for the Bakerloo Line Link is, in fact, not part of Crossrail but a London Underground project. Despite this it is being presented as a Crossrail project. Very fortuitously a worksite became available in a former Royal Mail sorting office in nearby London Street. This must rank as one of the most boring Crossrail worksites from the outside. Apart from the Crossrail signs there is little to indicate that this is anything other than an ordinary building being used in a perfectly ordinary way. The worksite will involve tunnelling that will not be used as part of the passageway. This work will probably not be wasted as, if a local development goes ahead, it will be incorporated into another entrance to the Bakerloo line at Paddington.

The Paddington Integrated Plan starts to disintegrate

Earlier on it was mentioned that work at Paddington was treated as an integrated project. With the coming of the Bakerloo Line Link this is starting to no longer be the case. The shell of the Bakerloo Line Link within the station box has been completed and, regardless of what the signs portray, it is the job of London Underground to complete the passages on the other side of the temporary concrete wall signifying the demarcation line.

The station box is one combined worksite at present but eventually the day will come when it gets handed over to its new owners. Network Rail, being in charge of the running tunnels, will be in charge of the fans and TfL Rail will take ownership of the new Crossrail station, but there will be further demarcation lines added to Network Rail’s station concourse and, of course, London Underground.

It will be interesting to see if TfL has a scheme similar to their in-house One Liverpool Street concept. The goal of One Liverpool Street is to integrate station operations as much as possible and, as far as the customer is concerned, create a single concern. Most of it is about training the staff to be familar with all the services offered and the station and not to restrict them to their own area of responsibility.

So much to do, so little time to do it in

We are now just three years away from Crossrail opening in central London. The final year is devoted to systems integration testing with, it is believed, the final three months devoted to test running of trains. So really, all the stations and the railway should, ideally, be completed in around two years. Crossrail has stuck to its schedule so far and there is no reason to believe that they will not continue to do so. Nevertheless there is still an awful lot to do at the stations, and Paddington is no exception.

Thanks to Andrew Dempsey at Crossrail for arranging the site visit and Pete Jarman for showing us around

Written by Pedantic of Purley